Light emitting diode and method of making the same

ABSTRACT

A light emitting diode (LED) and method of making the same are disclosed. The present invention uses a layer of transparent adhesive material to bond a transparent substrate and a LED epitaxial structure having a light-absorbing substrate. The light absorbing substrate is then removed to form a LED having a transparent substrate. By the use of the transparent substrate, the light emitting efficiency of the LED can be significantly improved.

FIELD OF THE INVENTION

[0001] The present invention relates to structure and method of making alight emitting diode (LED) chip, and more particularly to structure andmethod of making an AlGaInP LED chip.

BACKGROUND OF THE INVENTION

[0002] The conventional AlGaInP LED, as shown in FIG. 4, has a doubleheterostructure (DH), which is consisted of an n-type(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P lower cladding layer 4 with an Alcomposition of about 70%˜100%, formed on an n-type GaAs substrate 3, an(Al_(x)Ga_(1−x))_(0.05)In_(0.5)P active layer 5, a p-type(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P upper cladding layer 6 with an Alcomposition of about 70%˜100% and a p-type high energy gap GaAsP, InGaP,AlGaP, GaP, or AlGaAs current spreading layer 7. The emitting wavelengthof the conventional LED structure can be changed by adjustingcomposition of the active layer to generate a wavelength changed from650 nm red to 555 nm pure green. One disadvantage of the conventionalLED is that, when the light generated by the active layer is emitteddownward to the GaAs substrate, the light will be absorbed by the GaAssubstrate since the GaAs substrate has a smaller energy gap.

[0003] Accordingly, the light-output performance of the LED will begreatly reduced.

[0004] There are some conventional LED technologies have been disclosedin order to avoid the absorption of light by the substrate. However,these conventional technologies still have some disadvantages andlimitations. For example, Sugawara et al. disclosed a method, which hasbeen published in Appl. Phys Lett. Vol. 61, 1775-1777 (1992), thatadding a distributed bragg reflector (DBR) layer on the GaAs substrateso as to reflect the light emitted downward to the GaAs substrate and todecrease the light absorbed by the GaAs substrate. However, because theDBR layer only reflects the light that is of near normal incidence tothe GaAs substrate, so that the efficiency is not very great.

[0005] Kish et al. disclosed a wafer-bonded transparent-substrate (TS)(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P/GaP light emitting diode [Appl. PhysLett. Vol. 64, No. 21, 2839 (1994); Very high-efficiency semiconductorwafer-bonded transparent-substrate (Al_(x)Ga_(1−x))_(0.5)In_(0.5)P/GaP].This TS AlGaInP LED was fabricated by growing a very thick (about 50 μm)p-type GaP window layer using hydride vapor phase epitaxy (HVPE). Beforebonding, the n-type GaAs substrate was selectively removed usingchemical mechanical polishing and etching techniques. The exposed n-type(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P cladding layers are subsequentlywafer-bonded to 8-10 mil thick n-type GaP substrate. The resulting TSAlGaInP LED exhibits a two fold improvement in light output compared toabsorbing substrate (AS) AlGaInP LED. However, the fabrication processof TS AlGaInP LED is too complicated. Therefore, it is difficult tomanufacture these TS AlGaInP LEDs in high yield and low cost.

[0006] Horng et al. reported a mirror-substrate (MS)AlGaInP/metal/SiO₂/Si LED fabricated by wafer-fused technology [Appl.Phys Lett. Vol. 75, No. 20, 3054 (1999); AlGaInP light-emitting diodeswith mirror substrates fabricated by wafer bonding]. They used theAuBe/Au as the adhesive to bond the Si substrate and LED epilayers.However, the luminous intensity of these MS AlGaInP LEDs is about 90 mcdwith 20 mA injection current and is still 40% lower than the luminousintensity of TS AlGaInP LED.

SUMMARY OF THE INVENTION

[0007] As described above, the conventional LED has many disadvantages.Therefore, the present invention provides a LED structure and method ofmaking the same to solve the conventional disadvantages.

[0008] The present invention provides a light emitting diode. A lightemitting diode, the light emitting diode comprises a LED epitaxialstructure having a multi-layered AlGaInP epitaxial structure formed on alight-absorbing substrate; a transparent substrate; and a layer oftransparent adhesive material for bonding the transparent substrate andthe multi-layered AlGaInP epitaxial structure. The active layer of theLED can be composed of single heterostructure (SH), doubleheterostructure (DH), multi quantum wells (MQWs), or quantum wellsheterostructure (QWHs). Meanwhile, a first and a second ohmic contactmetal layer are connected to a first and a second conductive-typeepitaxial layers, respectively. Besides, both the first and second ohmiccontact metal layers are located on the same side.

[0009] The present invention provides a method for manufacturing a lightemitting diode, which comprises the steps of: providing a LED epitaxialstructure having a multi-layered AlGaInP epitaxial structure formed on alight-absorbing substrate; providing a transparent substrate; and usinga layer of transparent adhesive material, for example, SOG or silicone,to bond the transparent substrate and the multi-layered AlGaInPepitaxial structure. The light-absorbing substrate is then removed toexpose the first conductive-type etching stop layer so that a firstohmic contact metal layer is, for example, formed. The etching step alsoexposes the second conductive type epitaxial layer to form a secondohmic contact layer. In addition, both the first and second ohmiccontact metal layers are located on the same side.

[0010] An advantage of the present invention is to provide a simple LEDstructure, the adhesion process of the LED structure can be performed atlower temperature to avoid the evaporation problem of V group elements.Moreover, by the use of the transparent substrate, the light emittingefficiency of the LED can be significantly improved.

[0011] An advantage of the present invention is the simplified process,wherein the low cost glass can be used as the material of thetransparent substrate. Accordingly, a throughput with high yield and lowcost is achieved.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0012] FIGS. 1-3 are schematic, cross-sectional views of the process formanufacturing a light emitting diode in a preferred embodiment accordingto the present invention;

[0013]FIG. 4 is a schematic, cross-sectional view of structure ofconventional light emitting diode;

[0014] FIGS. 5-6 are schematic, cross-sectional views of structures oflight emitting diode of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0015] The present invention discloses a LED structure and method ofmaking the same and will be described in detail as below.

[0016] Referring to FIG. 1, the epitaxial structure of light emittingdiode of the present invention is composed of an n-type GaAs substrate26, an etching stop layer 24, n-type (Al_(x)Ga1−x)_(0.5)In_(0.5)P lowercladding layer 22 and (Al_(x)Ga1−x)_(0.5)In_(0.5)P active layer 20,p-type (Al_(x)Ga1−x)_(0.5)In_(0.5)P upper cladding layer 18, and p-typeohmic contact epitaxial layer 16.

[0017] In the above description, the material of the p-type ohmiccontact epitaxial layer can be AlGaAs, AlGaInP, or GaAsP, as along asthe energy gap of the material is larger than that of the active layer,and no light emitted from the active layer is absorbed.

[0018] Moreover, the active layer has an Al composition of about0≦x≦0.45, the lower cladding layer has an Al composition of about0.5≦x≦1, the upper cladding layer has an Al composition of about0.5≦x≦1. If x=0, then the composition of the active layer isGa_(0.5)In_(0.5)P, and the wavelength λd of the LED is 635 nm.

[0019] In the above description, the ratio of the compound such as(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P is a preferred example, the invention isalso applied to any ratio of III-V semiconductor material. In addition,the structure of the AlGaInP active layer 20 of the invention could be aSH structure, a DH structure, a multiple quantum wells (MQWs) structure,or a quantum wells heterostructure (QWHs). The DH structure comprisesthe n-type (Al_(x)Ga_(1−x))_(0.5)In_(0.5)P lower cladding layer 22, a(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P active layer 20 and a p-type(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P upper cladding layer 18, as shown inFIG. 1, wherein the preferred thickness of the lower cladding layer 22,the active layer 20 and the upper cladding layer 18 are about 0.5˜3.0,0.5˜2.0 and 0.5˜3.0 μm, respectively.

[0020] The preferred material of the etching stop layer 24 of theinvention can be any III-V compound semiconductor material that has alattice matched/or mismatched with that of the GaAs substrate 26. Thematerial of the etching stop layer 24 of the invention also has anetching rate much smaller than that of the GaAs substrate 26. Forexample, InGaP or AlGaAs can be good candidates of the etching stoplayer 24. In addition, the n-type AlGaInP lower cladding layer has anetching rate much smaller than that of the GaAs substrate. Therefore, ifthe lower cladding layer has enough thickness, an optional epitaxiallayer, which is used as an etching stop layer, with differentcomposition is not necessary.

[0021] The structure as shown in FIG. 2 comprises a transparent adhesivelayer 14, for example, spin on glass (SOG) and a transparent substrate(TS) 10. The material of the adhesive layer 14 is not limited to SOG.Any adhesive material with similar property, such as polyimide orsilicone is also applicable to the invention. The transparent substratecan be composed of glass, sapphire wafer, SiC wafer, GaP wafer, GaAsPwafer, ZnSe wafer, ZnS wafer, or ZnSSe wafer. These materials can bechosen as the transparent substrate as long as the light absorbed by thematerial is minor. One advantage of the present invention is that thetransparent substrate not must be single crystal wafer. The transparentsubstrate is used for supporting the LED epitaxial layer to avoid thisepitaxial layer from breaking, the current does not flow through thetransparent substrate. In other words, both the polycrystal andamorphous crystal can be used as the substrate. Accordingly, themanufacture cost is significant decreased.

[0022] Thereafter, the epitaxial layer structure of FIG. 1 is bondedtogether with the transparent substrate of FIG. 2. The adhesion can beperformed in a temperature, for example, 400° C., with pressure andheat, according to the method of the invention. A silicon oxide layercan be formed on the surface of the LED epitaxial and transparentsubstrate surface by, for example, deposition, evaporation, orsputtering, to improve the adhesion property between the LED epitaxialstructure and the transparent substrate. After that, a SOG layer iscoated, then a temperature, for example, 400° C., and a pressure areapplied for a period to the complete the adhesion between the epitaxialstructure and the transparent substrate. In order to provide betteradhesion, the LED epitaxial structure and the transparent substratebonded by the SOG layer, can be heated at a lower temperature, forexample, 50° C. to 300° C., to remove the organic solvent in the SOGlayer, and then the temperature is raised to a range between 300° C. and700° C. so that the bonding strength of the LED epitaxial structure, thetransparent substrate, and the SOG layer is excellent. Thereafter, theopaque n-type GaAs substrate is then removed by etchant, for example,5H₃PO₄:3H₂O₂:3H₂O or 1NH₄OH:35H₂O₂. However, the etching stop layer,InGaP or AlGaAs, still absorbs the light emitted from the active layer.Therefore, it is necessary to remove the etching stop layer and onlyremains a portion of this etching stop layer contacted with the n-typeohmic contact metal layer. A dry etching method, for example, RIE, isthen applied to remove portions of the n-type AlGaInP lower claddinglayer, AlGaInP active layer and p-type AlGaInP upper cladding layer tofurther expose the p-type ohmic contact epitaxial layer. A p-type ohmiccontact metal layer 28 is then formed on the p-type ohmic contactepitaxial layer 16. A n-type ohmic contact metal layer 30 is thereafterformed on the n-type AlGaInP lower cladding layer 22 to form a LEDstructure with p-type and n-type ohmic contact metal layers formed onthe same side, as shown in FIG. 3.

[0023] The light output power of the AlGaInP LED with wavelength 635 nmof the present invention is more than 4 mw (at 20 mA injection current)and is two times higher than the light output power of the conventionalabsorbing substrate AlGaInP LED.

[0024] The present invention is not limited to the AlGaInP LED havinghigh brightness, and is also suitable for other LED materials, forexample, red and infrared-red AlGaAs LED. The epitaxial structure shownon FIG. 5 is a cross sectional view of the second embodiment of thepresent invention. The AlGaAs red LED (650 nm) includes a stackedstructure of n-type GaAs substrate 51, n-type AlGaAs lower claddinglayer 52 with Al composition of about 70˜80% and thickness of 0.5 μm˜2μm, and a p-type AlGaAs upper cladding layer 54 with Al composition ofabout 70˜80% and thickness of 0.5 μm˜2 μm. The AlGaAs red LED structureis then bonded to a transparent substrate 56, for example, sapphire, bysilicone 55. The epitaxial structure is then etched by an etchant, suchas NH₄OH:H₂O₂=1.7:1 to remove the opaque n-type GaAs substrate.Thereafter, a wet etching or a dry etching is applied to remove portionsof the n-type AlGaAs lower cladding layer and AlGaAs active layer and tofurther expose the p-type AlGaAs upper cladding layer. A p-type ohmiccontact metal layer 57 is then formed on the p-type AlGaAs uppercladding layer 54. A n-type ohmic contact metal layer 58 is then formedon the n-type AlGaAs lower cladding layer 52 to form a LED structurewith p type and n-type ohmic contact metal layers formed on the sameside.

[0025] The light output power of the present invention AlGaAs LED is twotimes higher than the light output power of the conventional absorbingsubstrate AlGaAs LED. The AlGaAs LED of the present invention has awavelength 650 nm, but is not limited thereto.

[0026] The LED is composed of transparent substrate, and both the p-typeand n-type ohmic metal layer are formed on the same side of thetransparent substrate, therefore a flip chip package method can beapplied and the conventional wire bonding method is not necessaryanymore. Therefore, the LED formed by the method of the presentinvention has a better reliability. Furthermore, no light is absorbed bythe transparent substrate, therefore the brightness of the LED isimproved. In addition, the transparent substrate can be composed ofsapphire, glass or SiC with high hardness, therefore the thickness ofthe substrate can be down to 100 micrometers without breaking so that aLED structure with thin thickness and small size is manufactured.

[0027] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A light emitting diode, comprising: a LEDepitaxial structure having a multi-layered AlGaInP epitaxial structureformed on a light-absorbing substrate; a transparent substrate; and atransparent adhesive material for bonding the transparent substrate andthe multi-layered AlGaInP epitaxial structure.
 2. The light emittingdiode according to claim 1, wherein the light-absorbing substrate isGaAs.
 3. The light emitting diode according to claim 1, wherein the LEDepitaxial structure is an AlGaInP homostructure.
 4. The light emittingdiode according to claim 1, wherein the LED epitaxial structure is anAlGaInP heterostructure.
 5. The light emitting diode according to claim1, wherein the LED epitaxial structure is an AlGaInP doubleheterostructure.
 6. The light emitting diode according to claim 1,wherein the LED epitaxial structure is an AlGaInP quantum well.
 7. Thelight emitting diode according to claim 1, wherein the transparentadhesive material is SOG.
 8. The light emitting diode according to claim1, wherein the transparent adhesive material is polyimide.
 9. The lightemitting diode according to claim 1, wherein the transparent adhesivematerial is silicone.
 10. The light emitting diode according to claim 1,further comprises the step of removing the light-absorbing substrateafter the bonding of the transparent substrate and the LED epitaxialstructure.
 11. The light emitting diode according to claim 1, whereinthe transparent substrate is sapphire.
 12. The light emitting diodeaccording to claim 1, wherein the transparent substrate is glass. 13.The light emitting diode according to claim 1, wherein the transparentsubstrate is GaP or GaAsP.
 14. The light emitting diode according toclaim 1, wherein the transparent substrate is ZnSe, ZnS or ZnSSe. 15.The light emitting diode according to claim 1, wherein the transparentsubstrate is SiC.
 16. The light emitting diode according to claim 1,further comprises a silicon oxide layer formed on the surface of the LEDepitaxial structure and the transparent substrate.
 17. The lightemitting diode according to claim 1, wherein the transparent substrateand the multi-layered AlGaInP epitaxial structure are bonded by thefollowing stages: first stage: performing a heating and pressing step ina temperature between 50° C. and 300° C.; second stage: performing aheating and pressing step in a temperature between 300° C. and 700° C.18. A method of making a light emitting diode, comprising: providing aLED epitaxial structure having a multi-layered AlGaAs epitaxialstructure formed on a light-absorbing substrate; providing a transparentsubstrate; and using a transparent adhesive material to bond thetransparent substrate and the multi-layered AlGaAs epitaxial structure.19. The method according to claim 18, wherein the light-absorbingsubstrate is GaAs.
 20. The method according to claim 18, wherein the LEDepitaxial structure is an AlGaAs homostructure.
 21. The method accordingto claim 18, wherein the LED epitaxial structure is an AlGaAsheterostructure.
 22. The method according to claim 18, wherein the LEDepitaxial structure is an AlGaAs double heterostructure.
 23. The methodaccording to claim 18, wherein the LED epitaxial structure is an AlGaAsquantum well.
 24. The method according to claim 18, wherein thetransparent adhesive material is spin on glass (SOG).
 25. The methodaccording to claim 18, wherein the transparent adhesive material ispolyimide.
 26. The method according to claim 18, wherein the transparentadhesive material is silicone.
 27. The method according to claim 18,further comprises the step of removing the light-absorbing substrateafter the bonding of the transparent substrate and the LED epitaxialstructure.
 28. The method according to claim 18, wherein the transparentsubstrate is sapphire.
 29. The method according to claim 18, wherein thetransparent substrate is glass.
 30. The method according to claim 18,wherein the transparent substrate is GaP or GaAsP.
 31. The methodaccording to claim 18, wherein the transparent substrate is ZnSe, ZnS orZnSSe.
 32. The method according to claim 18, wherein the transparentsubstrate is SiC.
 33. The method according to claim 18, furthercomprises the step of forming a silicon oxide layer on the surface ofthe LED epitaxial structure and the transparent substrate.
 34. Themethod according to claim 18, wherein the step of bonding thetransparent substrate and the multi-layered AlGaAs epitaxial structureis performed in the following steps: first stage: performing a heatingand pressing step in a temperature between 50° C. and 300° C.; secondstage: performing a heating and pressing step in a temperature between300° C. and 700° C.